better error message with image tags containing upper-case letters

Created by: samuelkgutierrez

Hi,

I noticed this behavior when using master at 254fa07a.

Given the following command: ch-build -b ch-grow -t CapName -f /home/samuel/devel/bueno/examples/build-n-run/Dockerfile.mpich /home/samuel/devel/bueno/examples/build-n-run

I see the following error:

building with: ch-grow
error: image ref syntax, at end: CapName

May I suggest providing a nicer error message that points to the problematic tag name? I don't know if that's really the underlying problem here, but when I provide a tag name without capitalization (anywhere), things work as expected.

Thank you.